A conventional machine center such as CNC machinery includes a tool automatic changer located between the main shaft and the tool cabinet which received multiple tools for being picked up by the tool arm. The tool changer 10 as shown in FIG. 1 includes a motor 12 connected to the body 11 of the machine so as to drive a small befell gear 121 which drives a large bevel gear 13 and a cam 131 on the large bevel gear 13. The cam 131 is cooperated with a bearing 141 to drive a tube 14 which is connected to a sleeve 151 of a tool shaft 15 so that the tool shaft 15 can be indirectly driven. The large bevel gear 13 has a cam 132 which activates a swing arm 16 so as to move the tool shaft 15 axially in the tube 14. A sprocket 133 is connected to a side of an axle 17 of the large bevel gear 13 and a chain 134 is connected between the sprocket 133 and another sprocket 135 to rotate a signal wheel 18. A plurality of inducing members 19 are connected a frame 136 as shown in FIG. 2 so as to detect the angular positions and angles of the signal wheel 18 so as to control the rotation angle of the tool shaft 15. By controlling the angles and the travel of the tool shaft 15, the tool arm 152 at the lower end of the tool shaft 15 may catch or release the tools by two catch ends of the tool arm 152.
However, the power of the motor 12 is transferred to the tool shaft 15 via the cam 132, the bearing 141, and the sleeve 151 is low in efficiency and the sleeve 151 is difficult to be manufactured precisely as desired. Once the sleeve 151 is worn out, it has to be replaced with a new one. The cam 131 requires higher standard of machining to obtain the desired precision and this is costly. Furthermore, the transmission efficiency by the small and large bevel gears 121, 13 and the cam 131 is low. The signal wheel 18 is rotated by the motor 12 via the small bevel gear 121, the large bevel gear 13, the cam 131, the sprockets 133, 135 and the chain 134. Especially for the signal wheel 18, it requires a mechanism outside of the body 11 to install the signal wheel 18, such as a pivot shaft 181, a seal 182 and a cap 183 on the body 11. The whole mechanism is bulky and complicated. The manufacturing cost for the cam 131 is high and chain 134 needs to be adjusted manually and affects the precision of the signal wheel 18. The signal area 184 of the signal wheel 18 may not be matched with the inducing members 19 such that the tool shaft 15 cannot be precisely positioned. Besides, in order to smoothly operate the bevel gears 121, 13, lubrication oil is filled in the body 11. When moving the body 11, the lubrication oil could leak if the body 11 is put in a wrong orientation.